1. Field of the Invention
The invention relates to an optical arrangement, in particular a projection exposure apparatus for EUV lithography, comprising a housing that encloses an interior space; at least one, in particular reflective, optical element that is arranged in the housing, and at least one vacuum generating unit for generating a vacuum in the interior space of the housing. The invention further relates to a reflective optical element with a substrate and an electrically conductive multilayer system which on one of the sides facing the substrate comprises a reflective optical surface.
2. Background of the Invention
Optical arrangements in which optical elements are operated under vacuum conditions in an interior space of a housing are variously known. In EUV projection exposure apparatuses, typically, reflective elements, in particular mirrors, are used as optical elements, because at the wavelengths of approximately 13 nm as used in these applications no optical materials that provide adequate transmission are known. In such projection illumination instruments it is necessary to operate the mirrors in a vacuum because the service life of the multilayer mirrors is limited by contaminating particles or gases. To this effect it has been known to group the optical elements into three or more interconnected housing parts, which are divided by partition walls, namely a first housing part with a light source and a collector for focusing the illumination radiation, a second housing part with the illumination system, as well as a third housing part with the projection optics, the pressures within the housing parts being chosen to be different from each other, as described in detail in US 2005/0030504 A1, the entire contents of which are hereby incorporated by reference. In the context of this application, the term “housing” refers to both a housing of the EUV projection exposure apparatus overall, and to a partial region of the apparatus, in particular to one of the above-mentioned housing parts.
Such EUV projection exposure apparatuses or their individual housing parts are operated under vacuum conditions, wherein with the use of a dynamic gas lock (DGL) for reducing the outgassing products from the resist it is possible to achieve partial hydrogen pressures of 10−1 mbar and above, however usually (overall-) pressures of approximately 10−3 mbar and below are achieved. In the latter pressure region molecular movement is free, i.e. contaminating gases can propagate over the entire system. For this reason the achievable partial pressures of the contaminating gases are limited by all the components of the vacuum container, which components are present in the surroundings of the optical surfaces or further away from them. In the context of the present application, contaminating gases are defined as gases which are susceptible to form deposits on the optical surfaces, in particular when exposed to EUV-radiation. In this respect, non-volatile hydrocarbons having an atomic mass of 100 amu or above are treated to be contaminating substances, whereas volatile hydrocarbons in general having an atomic mass below 100 amu, e.g. methane (CH4), usually stay volatile even when irradiated with EUV light and consequently do not form deposits on the optical surface.
It is known that all types of atoms, molecules and compounds have a certain probability of reaching the optical surfaces and of adhering there. Together with the radiated-in EUV light and the photoelectrons generated as a result of this, in particular secondary electrons, there is a certain probability of these atoms, molecules or compounds reacting with the optical surface, which leads to an increase in contamination, increased damage and associated loss of reflection of the mirrors, and thus overall to transmission loss of the optics. Furthermore, adhering molecules can detach themselves from surfaces (e.g. chamber walls etc.) as a result of temperature increase, light radiation or electron radiation. Furthermore, contamination can also deposit on the optical surfaces in that thereon an excess charge is generated by the photoelectrons, in particular secondary electrons, which excess charge attracts electrically charged contaminating particles.
It is also known to at least partially remove contaminating substances which adhere to the optical surface by bringing the optical surface into contact with a cleaning agent, in particular with a cleaning gas. In such a way, carbon contamination can be removed from the optical surfaces by using atomic hydrogen as a cleaning gas. However, due to the fact that atomic hydrogen is highly reactive not only to carbon but also to other substances (especially metals by forming metal hydrides) which are present in the environment of the optical surfaces, contaminants which are released by these components by the cleaning itself may adhere to the optical surface.